______________________________________________________________________The molar amounts of defective sites (M d ) in several titanium(IV) oxide (TiO 2 ) powders were determined using photoinduced reaction of electron accumulation in deaerated aqueous solutions containing sacrificial hole scavengers and subsequent reduction of methylviologen to its cation radical. The measurements of pH dependence of typical anatase and rutile TiO 2 powders showed that these defective sites were of electronic energy just below the conduction band edge of TiO 2 in ranges of 0-0.35 V for anatase and 0-0.25 V for rutile. A linear relation of M d with the rate constant of electron-hole recombination determined by femtosecond pump-probe diffuse reflection spectroscopy revealed that M d could be a quantitative parameter of recombination between a 2 photoexcited electron and a positive hole. The tendency for M d to increase, but not proportionally, with increase in the specific surface area suggested that these defective sites are mainly located near the external surfaces of TiO 2 particles. A reciprocal correlation between photocatalytic activity for water oxidation in aqueous silver sulfate solution and M d revealed that the rate of recombination is one of the predominant physical properties governing the activities of TiO 2 powders in this reaction system. ______________________________________________________________________ IntroductionPolycrystalline titanium(IV) oxide (TiO 2 ) has been widely used as semiconductor photocatalysts in liquid-solid as well as in gas-liquid regimes 1-9 because of its sufficient photostability in various circumstances, nontoxicity for environments, and potent ability to induce various types of redox reactions. 10,11 It is well known that photocatalytic activity strongly depends on bulk and surface physicochemical properties of the TiO 2 powder, such as the kind of crystal structure, 12-15 surface area and particle size, [16][17][18][19][20] Experimental SectionMaterials. Several kinds of titanium oxide (TiO 2 ) powders, those from commercial sources (Degussa P-25, Merck, Ishihara CR-EL, and Hombikat UV-100), reference catalysts supplied from the Catalysis Society of Japan (JRC-TIO-1, -2, -3, and -5), and powders synthesized by the HyCOM method 6,28,29 followed by a calcination at 1073 K under O 2 (designated as HyCOM (1073)), were used. In a separate experiment to quantify the amount of accumulated electrons by another chemical reaction, a deaerated aqueous solution containing colloidal platinum particles was added to the photoirradiated suspension under Ar instead of the addition of MV 2+ . After shaking the mixture for a few minutes, the gas phase was analyzed using a gas chromatograph (GC, Shimadzu, GC-8A, equipped with an MS-5A column (GL Sciences) and a TCD detector). Measurement of the amount of accumulated electrons in photoirradiated TiOPP-DR spectroscopy. Details of PP-DR spectroscopy were reported previously 40,41 and are briefly summarized as follows. The light source consisted of a mode-lo...

Nanocrystalline anatase titanium(IV) oxide (TiO 2 ) particles were synthesized by hydrothermal crystallization in organic media (HyCOM) followed by calcination at various temperatures up to 1273 K, and they were characterized by analysis of surface adsorption of the substrates, as well as by X-ray diffraction (XRD) and Brunauer-Emmet-Teller (BET) surface area measurements. These HyCOM TiO 2 samples were used for three kinds of photocatalytic reactions: mineralization of acetic acid (AcOH) in aerated aqueous suspensions, dehydrogenation of 2-propanol (2-PrOH) by in situ platinized powders, and silver-metal deposition from silver ions (Ag + ) in deaerated aqueous suspensions of bare TiO 2 samples. Dependence of the photocatalytic activities on calcination temperature (T c ) and on the amount of adsorbed substrates in each reaction and correlations with the physical properties of HyCOM TiO 2 were examined. In the case of mineralization of AcOH, the activitiy of each sample was almost proportional to the amount of surface-adsorbed AcOH in the dark, and the uncalcined (as-prepared) HyCOM TiO 2 showed the highest activity, which was monotonically reduced with T c , that is, with decrease in the amount of surface-adsorbed AcOH. On the other hand, in the case of silver-metal deposition, the photocatalytic activity was enhanced by calcination at higher temperature, despite the simultaneous decrease in the amount of surface-adsorbed Ag + in the dark. Overall, the effects of calcination on the photocatalytic activities for several reaction systems strongly suggested that photocatalytic activity depends on two significant factors, adsorbability and recombination probability, corresponding to the specific surface area and crystallinity, respectively, and that the balance of these two factors determines the T c dependence.

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Nanocrystalline brookite-type titanium(IV) oxide (TiO 2 ) powder was synthesized by solvothermal treatment of oxobis(2,4-pentanedionato-O,O')titanium in a 1,2-ethanediol-water system in the presence of sodium acetate followed by hydrothermal treatment at 373 K in order to remove organic moieties contaminating the powder. The powder was calcined at various temperatures to change its physical properties and then used for three types of photocatalytic reaction: mineralization of acetic acid (AcOH) in an aerated aqueous suspension of bare TiO 2 powder, evolution of molecular hydrogen from 2-propanol in an aqueous suspension of in situ platinized powder, and formation of molecular oxygen (O 2 ) from silver sulfate in a deaerated aqueous suspension of bare TiO 2 powder.Dependence of the photocatalytic activities on calcination temperature (Tc) and correlations with the physical properties of brookite-type TiO 2 samples were examined. In the case of mineralization of AcOH, the uncalcined brookite TiO 2 sample having the largest surface area showed the highest rate of carbon dioxide evolution (R CO2 ), which was equal to that of representative 1/14Kominami et al., Photocatalysis of Brookite TiO 2 Submitted to Catalysis Letters commercial TiO 2 (Degussa P-25), and R CO2 decreased monotonously with Tc, i.e., with decrease in surface area. On the other hand, in the case of O 2 formation, the photocatalytic activity was enhanced by calcination at a higher temperature, despite the simultaneous decrease in surface area. Overall, the effects of calcination on the photocatalytic activities for the three reaction systems strongly suggested that photocatalytic activity of brookite-type TiO 2 depends on two significant factors, adsorbability and recombination probability, corresponding to the specific surface area and crystallinity, respectively, and that the balance of these two factors determines Tc dependence.